Newton's Second Law-Pulley problem

[Precursor]

Homework Statement

A 4.0-kg oak block on a horizontal, rough oak surface is attached by a light string that passes over a light, frictionless pulley to a hanging 2.0-kg object. The magnitude of the force of friction on the 4.0-kg block is 11.8 N.

http://img353.imageshack.us/img353/3743/pulleyai7.jpg​

Homework Equations

$$F_{net}$$=ma
$$F_{g}$$=mg

The Attempt at a Solution

I used $$F_{net}$$=$$F_{g}$$+$$F_{F}$$ and substituted in the values, but I don't seem to get the correct answer.

 

[Saladsamurai]

Either I am blind...or I don't see a QUESTION...

 

[dvdqnoc]

What are you looking for? Tension?

 

[dvdqnoc]

Assuming you're looking for tension...

http://www.geocities.com/chmode12/pulley.JPG

 

[Precursor]

I was looking for acceleration.

 

[dvdqnoc]

Ok. Well, do you see how you can use my demonstration to solve for acceleration as well? Put T on one side of both italicized equations, set them equal to each other, then solve for acceleration.

 

[temaire]

I am in the same class as Precursor, and I am also stuck on this question. I was able to get the acceleration, but I am unable to get the tension in the string. I got 15.7N. Dvd, you also got 15.7N, but our answer key for this assignment says that the answer is 17N. So, are we wrong or is our answer key wrong?

 

[Saladsamurai]

I am in the same class as Precursor, and I am also stuck on this question. I was able to get the acceleration, but I am unable to get the tension in the string. I got 15.7N. Dvd, you also got 15.7N, but our answer key for this assignment says that the answer is 17N. So, are we wrong or is our answer key wrong?

What did you calculate for an acceleration?

Casey

 

[temaire]

I calculated $$1.3 m/s^{2}$$ for the acceleration.

 

[dvdqnoc]

Perhaps your Force of Friction is incorrect? Double check and make sure all the numbers given is the same, and get back to me.

 

[chocokat]

Dvdqnoc - I apologize in advance in case I've missed something, but, on the right side of your calculations, you have

T - 11.8 = 2a

Should it be

T - 11.8 = 4a

since that block has a mass of 4g?

 

[lightsyrup]

Thank you - this was kind of helpful :)